The invention relates to a rolling rod as an inner tool in the production of seamless metallic hollow bodies, in particular stretch-forming of metallic hollow blocks to form seamless pipes by means of a multiple-stand rolling mill, having a surface comprising a nitriding layer. The invention also relates to a method for producing a seamlessly hot-rolled, metallic hollow body, in particular a steel pipe, in which a previously produced hollow block is subjected to a stretch-forming procedure in a multiple-stand rolling mill by means of a rolling rod in accordance with the present invention which is threaded therein, and prior to the commencement of rolling, i.e. the beginning of threading the rolling rod into the hollow block, the rolling rod is provided with a liquid lubricant and which is subsequently dried.
After the invention by the Mannesmann brothers of producing a thick-walled hollow block pipe from a heated block by means of rolling, there have been various proposals for stretch-forming this hollow block pipe in the same heat in a further heat working step, upon which in a third rolling step the outer diameter is reduced to the finished diameter of the rolling mill. Keywords in this regard include the back-step method, push bench method, plug rolling method and rod rolling method. In the first rolling step, a rotary forge mill produces a so-called hollow block from a block which is generally solid. In exceptional cases, pre-bored blocks can also be used instead of solid blocks. In the second rolling step, the hollow block is stretch-formed, wherein nowadays multiple-stand rod rolling mills are predominantly used for this purpose. The stretch-forming of the hollow block, which enters the rolling mill and is at a temperature of about 1000 to 1200° C. is effected by means of a rolling rod. For this purpose, the rolling rod is threaded into the hollow block and for the most part is conveyed by a driven transport roller to the first rolling stand. The number of rollers per stand is typically at least two, but nowadays it is often three, wherein the process sequence is as follows.
Rod rolling mills are distinguished on the one hand according to the manner in which the rod speed is controlled, and on the other hand according to the number or rollers per stand, wherein two or three rollers can be provided. A plurality of stands are always connected one behind the other. In the case of the method variant considered in this case, the rolling rod moves during the actual rolling procedure at a constant speed through the rolling mill. For this purpose, the rolling rod must be retained by an electromechanical system and also guided in a controlled manner to achieve the constant rolling rod speed. The retention system is referred to as a retainer. In order to ensure that, after the rolling rod has been threaded into hollow block during longitudinal stretch-forming, the hollow block to be rolled can slide on the rolling rod during rolling, the rolling rod must be provided beforehand with a lubricant. For this purpose, a graphite-containing lubricant is typically sprayed in liquid form onto the rolling rod and is dried at temperatures of 80 to 130° C. At lower temperatures the lubricant does not completely dry out in a reliable manner and at higher temperatures the so-called Leidenfrost effect occurs, as a result of which a uniform layer is not created and parts of the surface remain unlubricated. Therefore, the attempt is to perform the drying at temperatures below 100° C.
In addition to lubricating the rolling rod, in accordance with European patent specification EP 1 775 038 B1 the hollow blocks to be rolled are sprayed internally with a deoxidant (e.g. borax) prior to the stretch-forming procedure in the rod rolling mill, in order to dissolve the oxide scale which results after rolling of the hollow block, wherein the dissolved oxide scale acts like additional lubrication. A disadvantage in this case is the use of borax which can be have a gene-altering effect and therefore should not be used.
The hollow block located on the rolling rod is then rolled out in the rod rolling mill to form a so-called mother pipe. The term “mother pipe” is used because in the third rolling step pipes having various finished diameters can be produced from the same mother pipe dimension by means of size rolling or stretch reducing. The rolling rod itself generally consists of a working part and a blank part. The blank part is required in order to bridge distances which are necessary in terms of process technology. Therefore, rolling does not take place on the blank part of the rolling rod. For the sake of simplicity, the term “rod length” will be used hereinafter to refer to the length of the working part.
Although highly heat-resistant steels based on chromium-molybdenum are typically used as the rod material, in particular the materials which are difficult to form cannot be rolled without encountering problems. Such materials frequently contain chromium, such as e.g. 100 Cr6 or the corrosion-resistant steels for the energy sector having chromium contents of greater than 5 wt %. The rolling rod is subjected to dispropor-tionate thermal loading and abrasion by these materials and the service life of the rolling rod is considerably reduced. This problem increases as the chromium content of the steels increases. Moreover, the risk of defects on the inner surface of the pipes caused by prematurely worn rolling rods is considerably increased.
In order to minimise the coefficient of friction, it is known from German laid-open document DE 37 42 155 A1 to chromium-plate the rolling rod surface and to apply a lubricant. However, chromium-plating releases toxic chromium IV which is harmful to the environment, for which reason alternative solutions have been sought.
In order to increase the service life of these rolling rods, which are subjected to extensive thermal loading and abrasion, during rolling in particular of chromium-containing steels and in order to minimise friction-induced defects on the inner surfaces of the pipes, it is known from Japanese laid-open document JP 06262220 A to chromium-plate the rolling rods instead with a nitriding layer which has a roughness of 0.5 to 5.0 μm. Details relating to required layer thicknesses are not provided.
In Japanese laid-open document JP 2009045632 A, the nitrided layer is supplemented by an oxide layer, to be applied thereon, in order to ensure that the service life of the rolling rod is impressive. The layer thickness of the nitriding layer is stated as being 50-500 micrometers, and that of the oxide layer is stated as being 3-20 micrometers.
Tests have shown that although nitrided surfaces have emergency operating characteristics when the lubricating film tears off during rolling, the service life of the rolling rods treated in this manner still cannot be significantly improved and defects on the inner surfaces of the pipes still cannot be significantly reduced.
The difficulty in using surface-nitrided rolling rods which are known per se resides in adapting the entire process, from producing the rolling rods, pre-treating the hollow block, lubricating the rod to the rolling process itself, to suit each other such that an assured rolling process is guaranteed for steel pipes consisting of unalloyed steel grades right through to chromium-containing steels which are difficult to hot-form.
Furthermore a rolling mandrel is already known form the German patent document DE 197 14 317 C1 which is provided with a surface layer via a PVD (physical vapour deposition) method on the basis of CrN.